(1) Field of the Invention
The present invention relates to a method of producing hydrogen-absorbing alloy for use in negative electrodes of nickel-hydrogen alkaline storage cells.
(2) Description of the Prior Art
Hydrogen-absorbing alloy is capable of reversibly absorbing and desorbing hydrogen and is therefore employed for the negative electrode material in nickel-hydrogen alkaline storage cells. Nickel-hydrogen alkaline storage cells employing hydrogen-absorbing alloy have many advantages over conventional storage cells such as lead storage cells and nickel-cadmium storage cells, the advantages including greater energy density. For this reason, it is believed that nickel-hydrogen alkaline storage cells will account for a large proportion of next generation alkaline storage cells.
Nickel-hydrogen alkaline storage cells, however, have a drawback that the high-rate discharge characteristic at low temperature is insufficient. In order to improve the high-rate discharge characteristic, there has been suggested a technique of subjecting hydrogen-absorbing alloy to a heat-alkali treatment. In addition, in order to further improve this technique, Japanese Unexamined Patent Publication No. 9-283130 suggests a method in which a reducing agent is added to the alkali treatment solution, and Japanese Unexamined Patent Publication No. 7-326353 suggests a method in which metal ions are added to the alkali treatment solution.
The foregoing improved methods, however, do not take the pH of the alkali treatment solution into account. Therefore, the advantageous effects of adding a reducing agent or metal ions are not fully utilized. As a consequence, the foregoing methods have not yet sufficiently improved the high-rate discharge characteristic at low temperature.
In view of the foregoing and other problems in prior art, it is an object of the present invention to provide a method of producing a hydrogen-absorbing alloy that is capable of remarkably improving low-temperature high-rate discharge characteristics of nickel-hydrogen alkaline storage cells by sufficiently utilizing the effects of adding a reducing agent and/or metal ions.
This and other objects are accomplished in accordance with the present invention by providing a method of producing a hydrogen-absorbing alloy for use in a nickel-hydrogen alkaline storage cell, comprising the steps of: (a) immersing a hydrogen-absorbing alloy in an alkali treatment solution having a temperature of 60xc2x0 C. or higher to subject a surface of the hydrogen-absorbing alloy to an alkali treatment; (b) after the step (a), adding a pH-adjusting agent and a reducing agent to the alkali treatment solution to treat the surface of the hydrogen-absorbing alloy using the alkali treatment solution; and (c) washing the hydrogen-absorbing alloy treated by the step (b).
According to the above-described method, in the step (a), metal oxide or the like on the surface of the alloy is dissolved in the solution. In the step (b), the surface of the alloy is reduced, and also, metal ions dissolved in the solution in the step (a) are formed in the state of metal and deposited on the surface of the alloy. In the step (c), the solution is removed from the surface of the alloy. Thereby, it is made possible to produce a hydrogen-absorbing alloy excellent in electrochemical activity and electrical conductivity. It is noted here that in the step (b), a pH-adjusting agent in addition to a reducing agent is added to the alkali treatment solution in order to deposit metal on the surface of the alloy. This results in a high efficiency in reduction since the reduction is performed at an appropriate pH value. Accordingly, the surface of the alloy is sufficiently reduced, while metal ions dissolved out from the surface of the hydrogen-absorbing alloy into the solution in the step (a) are formed in the state of metal and sufficiently deposited on the surface of the hydrogen-absorbing alloy. Therefore, electrical conductivity of the surface of the hydrogen-absorbing alloy is improved while electrical resistance between the alloy particles each other is decreased. As a result, the nickel-hydrogen alkaline storage cell employing such a hydrogen-absorbing alloy achieves a remarkably improved high-rate discharge characteristic at low temperature.
In the step (b) of the above-described method, the alkali treatment solution may have a pH of 4 to 9 after the pH-adjusting agent is added.
When the pH of the alkali treatment solution is in the range of pH 4 to pH 9, metal is more efficiently deposited from the alkali treatment solution onto the surface of the alloy, and as a result, the electrical conductivity of the alloy is further improved.
In the above-described method, the alkali treatment solution may contain a complexing agent.
When the alkali treatment solution contains a complexing agent, metal ions dissolved in the solution in the step (a) become a complex compound, and therefore metal does not easily deposit as a hydroxide. As a result, more metal ions are present in the solution after the step (a), and the ions are reduced in the step (b) and deposited on the surface of the alloy. Consequently, the amount of metal on the surface of the alloy is increased and the electrical conductivity between the alloy particles each other is thereby improved. Hence, the low-temperature high-rate discharge characteristic of the cell is further improved.
In the above-described method, the alkali treatment solution may contain metal ions.
By adding metal ions to the alkali treatment solution in advance, the amount of metal deposited on the surface of the electrode in the step (b) can be further increased.
As discussed above, by sufficiently utilizing the effects of adding a reducing agent and/or metal ions, the present invention can achieve remarkable improvement in high-rate discharge characteristics of nickel-hydrogen alkaline storage cells at low temperature.